It is common in the petroleum industry to employ foamed liquids in cleaning out and/or fracturing wells, particularly petroleum wells with low formation pressures and gas wells. Foam is employed for a number of reasons, among them the necessity to avoid formation damage from the large hydrostatic head generated by a column of liquid in the well bore, and the desirability to conserve liquid well treating materials, the quantity thereof required being significantly reduced by foaming.
Several methods are known for generating foam for well treatment, such as generating the foam at the well head by using a TEE or pumping the treating liquid down a well bore annulus to the treatment zone while pumping a gaseous component of the foam down a tubing string which ends at the treatment zone. The former method requires very high pumping pressures in deeper wells, due to the frictional losses incurred by pumping foam. In a well with a high fracture gradient, defined as treatment pressure over treatment zone depth, such frictional losses may render foam fracturing prohibitively expensive or impossible to perform. The latter method produces an unstable foam with non-uniform bubble size and dispersion as well as "slugging" of gas and liquid, if in fact a foam per se is produced at all.
U.S. Pat. No. 3,889,764, issued June 17, 1975 to John G. Jackson, discloses a method and apparatus for producing foam downhole in a drill string, in which the liquid and gaseous components of the foam may be mixed at the surface and pumped down the string to the foam delivery apparatus above the drill bit, or may be routed to the bottom of the string through separate conduits. The foam delivery apparatus is opened and closed by a pressure-controlled valve, and the foam is generated through impingement of the gas/liquid mixture onto a knife edge located between the discharge orifices of the apparatus. The foam thus generated travels up the wellbore carrying the material dislodged by the drill bit. When the liquid and gas are mixed at the surface, high pumping pressure is required. If several conduits are employed to carry the separated components to the drill bit, this requires the unwieldy handling of both the drill pipe and two inner conduits. In either case, the knife edge generator is unsuitable for use with abrasive-laden fluids, and the use of a spring loaded control valve requires a fairly precise estimate of the foam flow desired at any given time, in order to precalculate the desired pressure drop and adjust the valve spring.